The Abbasid Caliphate overtook the Umayyad caliphate. The second abbasid caliph, created a new office in government, called the vizier, who was essentially a deputy or very powerful advisor, second hand to the caliph. The abbasids also expanded trade networks, an built a very powerful empire. http://www.islamicity.com/education/ihame/default.asp?Destination=/education/ihame/6.asp

The Abbasid Caliphate was the third of the Islamic caliphates. It was ruled by the Abbasid dynasty of caliphs, who built their capital in Baghdad after overthrowing the Umayyad caliphate from all but the al-Andalus region. Approx 6th - 8th century. AD

Abbasid Caliphate- the Abbasid caliphate was the third of the Islamic caliphates to succeed the Prophet Mohammed. The Abbasid dynasty descended from the prophets youngest uncle. They ruled as caliphs from their capital in Baghdad, modern Iraq, after taking authority over the Muslim empire from the Umayyads. The first centered government was in Kufa.